Help and explanation finding the duration of the portfolio with the stated securities

What is the duration of a portfolio consisting of the following securities? 1. A 5-year zero, $1,000 worth purchased 2. An 15-year coupon bond with 9-year duration, $4,000 worth purchased 3. A 9-year zero, $5,000 worth purchase Round your calculations to the nearest 0.1, i.e., one decimal place